In general, electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A solid polymer fuel cell is a specific type of fuel cell that employs a membrane electrode assembly (“MEA”), which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. Solid polymer fuel cells typically operate in a range from about 40° C. to about 150° C.
A broad range of reactants has been contemplated for use in solid polymer fuel cells and such reactants can be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may, for example, be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture. An advantage of liquid feedstocks and fuels, such as methanol, particularly in non-stationary applications, is that they are relatively easy to store and handle. Further, fuel mixtures that react directly at the anode in a direct liquid feed fuel cell avoid the use of a reformer in the fuel cell system.
A direct liquid feed fuel cell is a type of solid polymer fuel cell that operates using at least one liquid reactant stream. Most typically, direct liquid feed fuel cells operate directly on an organic liquid fuel stream typically supplied as a fuel/water vapor or as an aqueous fuel solution. Typically, methanol is used as the fuel in a direct liquid feed fuel cell though other organic fuels can be used such as, for example, ethanol or dimethyl ether. When methanol is used, the direct liquid feed fuel cell is often referred to as a direct methanol fuel cell (DMFC). The reaction at the anode involves the direct oxidation of methanol and water. There is often a problem in DMFCs with crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over typically reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, dilute solutions of methanol, for example, 5% methanol in water, are typically used as fuel streams. The fuel streams in DMFCs are usually recirculated in order to remove carbon dioxide, a by-product of the reaction at the anode, and to re-use the diluent and unreacted fuel in the depleted fuel stream exiting the DMFC. Methanol is added to the circulating fuel stream before it re-enters the fuel cell in order to compensate for the amount consumed, thereby providing a fresh mixture at the desired methanol concentration. Since the amount of methanol consumed is variable (depending on the load, crossover, and other operating parameters), the methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor, and fresh methanol is admitted by a fuel injector in accordance with the signal from the sensor.
Relevant factors in developing a system for controlling the concentration of methanol in the system include cost, size, simplicity, reliability, longevity, concentration range and dynamic response.